Migrate old project structures

[cmather]

As we move files around we need a way to upgrade from an existing project. This also applies to a barebones Meteor project (e.g. app.html, app.css, app.js) to an em/iron project structure.

[Sivli-Embir]

👍 I had to troubleshoot a project today using and old em version. Would have saved some time and led to less Collectionundefined files.

[cmather]

I created a migrate command but it's only from em -> iron. We should have a robust way of doing this from meteor -> iron, or from different versions of iron.